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Objective Reviews & Commentary - An Engineer's Perspective

February 9, 2011

Headphone Impedance Explained

INTRO: Headphones range in impedance from 16 ohms to 600 ohms. And some models even come in multiple impedances. So which impedance is best? The answer mostly depends on what you plan to plug them into.

WHAT’S BEST? Are lower impedance headphones better than higher impedance ones? What’s the best output impedance? This is over simplifying, but here are some general guidelines:

If you’re looking for headphones to use with a portable player or laptop, stick to the range of 16 – 32 ohms with a sensitivity (efficiency) rating of at least 100 dB/mW. There are some higher impedance headphones, up to 80 ohms or so, that are efficient enough to work well with at least some portable gear—especially if you don’t like it very loud. But, in general, the lower the impedance the better the match with battery powered devices.

If you don’t know the output impedance of your source device, it’s best to avoid balanced armature headphones as they can interact with a higher output impedance in some ugly ways that make them sound substantially worse.

If your source complies with the European maximum volume regulations (many new phones do even when sold outside of Europe) it is even more important to choose headphones with, ideally, a 16 ohm impedance at at least 100 dB/mW sensitivity. The European compliant devices have even less maximum output than most other portable gear.

If you’re using a dedicated headphone amp or DAC, check the manufacture’s specifications or guidelines for the recommended headphone impedance range.

If you’re looking at headphones with less than 100 dB/mW check out at least the first few paragraphs of the More Power article.

DIFFERENT HEADPHONE IMPEDANCES: In the world of consumer speakers, nearly all have an impedance in a relatively narrow range of 4 – 8 ohms. This makes it easier for designers of amplifiers, receivers, etc. as they pretty much know what the speaker impedance will be. With headphones, however, things are a lot different. There are few well defined standards, so impedances vary widely between manufactures and headphones designed for portable, home and studio/professional use. The range from 16 ohms to nearly 40 times higher at 600 ohms. This creates lots of compatibility issues.

WHY IMPEDANCE MATTERS: Headphone sources generally put out very different amounts of power into different headphone impedances. For example the Clip+ portable player can put out 16 mW into 16 ohms but only 0.8 mW into 300 ohms. The FiiO E7 can put out over 100 mW into 16 ohms but only 2.8 mW into 600 ohms. And some sources are not as compatible with low impedances such as the Mini3 and FiiO E9.

LOUD ENOUGH: Most headphone sources can manage at least 5 mW of power into 16 ohms. With headphones rated for 100 dB/mW (the suggested minimum above for portable use) that means 107 dB SPL which is in the range of 105 dB SPL – 115 dB SPL that most consider “loud enough”. See: More Power

WHAT’S CONSIDERED “HIGH IMPEDANCE”? I’m not aware of any hard rule but generally it’s safe to say 100 ohms and higher qualifies as “high impedance”. Such headphones are usually not designed for portable use. And it’s safe to say 32 ohms and lower qualify as “low impedance” and typically work well for all applications. That leaves a gray area between 32 and 100 ohms where other factors determine how suitable the headphones are for a given source.

BALANCED ARMATURE IEMs: Many of the high-end in-ear monitor type headphones use a technology known as balanced armature (also called “micro armature”, etc.). Some examples including popular in-ear models from Shure, Etymotic, Ultimate Ears, etc. These headphones usually have a rated impedance between 16 and 32 ohms but their actual impedance typically varies greatly with frequency. The 21 ohm Ultimate Ears SuperFi 5, for example, ranges from 10 ohms to 90 ohms. These wide variations interact in often unfavorable ways with the Output Impedance of the source.

WHY DO HEADPHONE IMPEDANCES VARY SO MUCH? There are multiple reasons:

History Favored High Impedance - Before portable nice sounding portable gear came along most high quality headphones were plugged into either home stereo equipment or professional studio equipment. Because it was cheap and easy, those headphone jacks on receivers from the 60’s – 80’s typically had a very high output impedance. They simply used large resistors to drop the power from the speaker outputs.

Outdated 1996 Standard – A standard was set in 1996 for headphone output impedance to be 120 ohms—apparently more as a convenience for makers of gear with with headphone jacks. Stereophile has since summed up their opinion of the standard with: “Whoever wrote that must live in a fantasy world.” A much lower output impedance has significant advantages but the poorly conceived standard still likely had an influence towards higher impedance headphones.

High-End Headphones Favor High Impedance - There can be some significant advantages to higher impedance headphones. A higher impedance allows for more turns of wire to be used in the voice coil of the driver. This can result in a better “motor system” with fewer compromises resulting in better overall sound. Higher impedance headphones also require less current to drive and that typically reduces distortion in multiple ways. It makes them more immune to output impedance differences between sources and also less sensitive to long cables and sharing a common wire in 3 wire cables and 3 wire connectors (i.e. headphone plugs and jacks). The amplifiers driving headphone nearly always produce lower distortion into higher impedances.

The iPod Revolution Favors Low Impedance – As of 2009 over 220 million iPods have been sold. And before iPods there were other portable devices. It’s probably fair to say, if you include the headphone output on music-friendly cell phones, there are now over a billion portable audio devices in use. This is important because battery powered devices don’t work well with high impedance headphones. So they forced development of lower impedance headphones that would work well. But this was in direct conflict with the above three paragraphs. So you have high-end goals, and lots of history competing with the massive market potential of a billion portable devices.

WHAT HEADPHONES CAN I USE WITH MY SOURCE? This is main question most people have. It comes down to just three things:

Power Requirements – Will a given source have a enough power to drive a given pair of headphones to a loud enough volume level? The 100 dB/mW guideline mentioned earlier should get someone close. For headphones with a lower sensitivity (or no sensitivity) specification, see the More Power article.

Device Output Impedance – This is tricky because the output impedance of most sources is unknown. But the idea is to follow the “1/8th Rule” described in Output Impedance Explained. If you multiply the output impedance of your source by eight, that’s the lowest impedance headphones you should use with that source. The FiiO E9 amp, for example, has a 10 ohm output impedance. So it should only be used with headphones of 80 ohms or higher if you want to be assured of the best sound quality.

Source Distortion – Some sources have a hard time with lower impedance headphones. Tube amps with no output transformers (like the Bottlehead Crack), for example, have much higher distortion into low impedance headphones.

CAN A MISMATCH DAMAGE ANYTHING? Using the wrong headphones for a given device generally won’t harm anything besides the sound. But some sources are capable of very high output and might damage more sensitive headphones. But this is only an issue if you turn up the volume well past sane levels—which could happen accidentally. Some headphone amps have a gain switch to help prevent this problem.

WHEN IS A HEADPHONE AMP OR DAC REQUIRED? If a given pair of headphones will not play loudly enough, or has other obvious problems with a given source, then a headphone amplifier or DAC may help. But these devices can also create their own problems. And if a given source already has a low output impedance and enough output power, than adding an amplifier can often make things worse instead of better. See: Headphone Amps/Dacs Explained.

BOTTOM LINE: It’s best to follow the guidelines above unless you otherwise can be fairly certain a particular pair of headphones will work well with a particular source.

TECH SECTION:

VOLTAGE AND CURRENT: It’s important to have at least some understanding of voltage and current to understand impedance and what follows. Voltage is analogous to water pressure (i.e. PSI) while current is analogous to the volume of water (i.e. gallons per minute). If you let water run out of the end of your garden hose with nothing attached you get a lot of flow (current) and can fill a bucket quickly but the pressure at the end of the hose is near zero. If you put a small nozzle on the hose the pressure (voltage) is much higher but volume of water is reduced (it takes longer to fill the same bucket). The two are typically inversely related. High pressure usually means low flow and visa versa. The same is true of voltage and current.

HOSE NOZZLES: Impedance is roughly analogous to the size of a hose nozzle. A high impedance headphone is like a tiny nozzle. To get much water out you need a lot of pressure (voltage). A low impedance headphone is more like filling a bucket and requires more flow but not much pressure. A lot of headphone outputs on devices are good at one, OR the other, but not both. So it’s important to know which you have and match the headphone accordingly.

HEADPHONE LOAD IMPEDANCE: This article talks about two different kinds of impedance—the impedance of the headphones (usually easy to find) and the impedance of the source (usually difficult to find).A “perfect” source has an output impedance of zero ohms. This means it will always deliver the same output into any load. In practice, any output impedance below about 1 ohm approximates a “zero ohm” source. The blue circle on the left above represents a “perfect source”, the blue resistor (zig zag line) in the middle represents the output impedance. And the resistor on the right represents the load impedance (the headphones). If the output impedance is not zero, the voltage produced by the source will be reduced when a load is connected. The higher the output impedance, the greater the drop in voltage at the load. This drop is given by the formula: Load Voltage = Source Voltage * ( Load Resistance / ( Load Impedance + Output Impedance) ). For more information see Wikipedia Voltage Divider:

R vs Z:Resistance is measured in ohms and represented in electrical engineering by the letter R. It’s typically measured with a DC current and it’s what a Digital Multi-Meter measures. Impedance, however, is more complex and measured with an AC current and the letter Z is the correct representation (but some still use R). Because music is an alternating signal, not DC, impedance is how headphones are rated. Impedance is made up of resistance, capacitance and inductance. Few headphones “look” like perfect resistors—most have significant inductance and at least small amounts of capacitance. They’re what’s known as a reactive load and the math is considerably more complex than just the most basic form of Ohm’s Law.

MEASURING HEADPHONE IMPEDANCE: You can’t measure headphone impedance with a DMM (although many incorrectly try). The more reactive the headphones are, the less accurate the reading will be as the DMM is only measuring the DC resistance “R” not the AC impedance “Z”. The DC resistance will nearly always be lower than the AC impedance. And an AC impedance measurement is only valid at one particular frequency so it’s best to plot if versus frequency in a graph. To do this right requires specialized equipment that can monitor the current and voltage independently over the entire audio band. I use my Prism Sound dScope.

YOUR IMPEDANCE MAY VARY: Virtually all passive headphones (without their own powered electronics) have a varying impedance that changes with frequency. As described above, they don't behave like a simple resistance when driven with typical audio signals. The gold colored graph below shows the impedance, in ohms, of the Ultimate Ears SuperFi 5 Pro in ear monitors. You can see they’re only at their rated 21 ohm impedance below about 200 hz. The impedance rises to nearly 90 ohms at about 1200 hz and drops to below 10 ohms at 11,000 hz:

PHASE: When impedance varies with frequency so will the phase. In this case “phase” is the time difference between the peak voltage and peak current. The greater the phase variation the more “reactive” the load. Generally, the greater the phase shift, the more difficult the load is to properly drive. The white curve above is the phase shift shown in degrees. A simple resistor will have essentially zero phase shift with frequency over the audio band.

HEADPHONE DESIGN COMPROMISES: Drivers, in speakers or headphones, have moving mass. The diaphragm of the driver is suspended in a way that allows it to move. The stiffness of this suspension, the weight of the diaphragm, and sometimes the enclosure it’s in, work together to form a natural resonant frequency. The main resonance in full size headphones is usually in the bass frequencies and most high quality drivers require some sort of damping to control this resonance. If it’s not controlled, it tends to seriously degrade the bass performance of the driver. The impedance peaks at this resonance. Here’s the popular Sennheiser HD280 headphones showing an obvious bass resonance at 75 hz:

WHAT ABOUT TYPICAL EARBUDS? The vast majority of reasonably priced dynamic (not balanced armature) earbuds headphones have a 16 or 32 ohm nominal impedance that might vary by just 1 or 2 ohms. Here's the popular Sony MDR-EX51 which is mostly 17 ohms and rises to 18 ohms at its 5 khz resonance:

HIGH END HEADPHONES: Here’s the 300 ohm Sennheiser HD 650 which vary from about 305 ohms to 530 ohms:

OTHER RESOURCES: If you missed the links earlier, you might want to check out:

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comments:

I'm confused as heck. DIY folks like the sound of their higher impedance headphone cables (just some resisters between the cable sections). It seems to me frmo this article that such a practice is the same as increasing output impedance, which wouldn't be a good move.

So my questions are these:-Is it the same as increasing output impedance to have a higher resistance headphone cable?-Would it be better to have the resiters on the common/ground wires of the headphone cable instead? It seems this is never practiced.

See my answers to your comment in the other impedance article. I'm not aware of many "high impedance" headphone cables with series resistance added. But you generally want the resistance of the common wire to be as low as possible as even small amounts of resistance there will severely degrade the crosstalk.

Adding series resistance to the left and right channel will change the sound of many headphones. And some might even prefer the changed sound. But, in general, to get the most accurate sound you don't want to add resistance.

"A higher impedance allows for more turns of wire to be used in the voice coil of the driver."I do not understand this statement. As far as I know, the thinner the cable, the higher the resistance/impedance. So wouldn't a voice coil with thicker winding result in a lower impedance? Shouldn't two voice coils with the same winding thickness but different wire thicknesses have (roughly) the same impedance, all other things being equal?

There is the claim that, all other things being equal, higher impedance allows for better sound quality. For example, beyerdynamic offers its DT880 headphone in 32, 250 and 600 ohm variants, and many favor the 600 ohm version citing this claim. Is it true?

When you compare the architecture of the AKG K701 (62 ohm) with the K601 (120 ohm), their main difference are the types of wire being used for the voice coils; the K601 uses a conventional round-wire voice coil whereas the K701 features a flat-wire voice coil.AKG claims that "[f]lat, aluminum wire creates a stronger, more uniform magnetic field in the voice coil, enabling the K701s' diaphragm to track audio signals with unprecedented accuracy. It also allows for a lighter, smaller coil that increases motor efficiency and significantly lowers impedance, resulting in a soundstage of unequaled realism you'll notice, especially in the lower frequencies".While I can understand that a flat wire results in denser winding and thus a lower impedance and higher efficiency, all other things being equal, what about AKG's other, more esoteric claims? Is it true that a flat-wire voice coil allows for better sound quality, all other things being equal?

@inarc, more turns of wire on the same diameter voice coil generally means more magnetic force. But more turns also generally means higher impedance. It's the higher magnetic force that provides better sound quality. That's just one reason why higher impedance headphones, like the DT880-600, can sound better.

Flat wire lets you pack more wire into the same amount of space (round wire leaves air gaps between the turns in the coil). So it can increase the magnetic force for a coil of a given size. The coil could also be smaller for the same force. Making it smaller in diameter could save some weight from the former but you're mostly leaving out air.

Lower impedance does generally increase efficiency but it really depends on the driving source. The K701 is a relatively difficult to drive headphone as it needs relatively high amounts of current AND voltage. It's not especially sensitive or efficient.

A lot of what you quote from AKG is just the usual marketing hype (i.e. "unequaled realism") and difficult to validate or dispute. In general, flat wire allows for either a more powerful, or more compact, voice coil. So it does have some objective advantages. But the voice coil design is just one relatively small portion of headphone design. The magnetic system, diaphragm, suspension, damping and enclosure are all critical as well.

So, take a look at the impedance graph here:http://www.innerfidelity.com/images/SennheiserHD595.pdf

It looks like there's a huge (really huge) spike at around 100hz, as is pretty much the same as the graphs you've posted.

I have an old stereo receiver which has a headphone output which I suspect has a massively high output impedance. Well, when I plug my HD595s into them, the bass at right around the 100hz region (well, I presume it's around there) becomes extremely loose and undampened.

Is it the large output impedance of the source mixed with the high impedance at that frequency that is doing this?

@wcecsharp, That's very likely what's happening. That 100 hz peak is the natural resonance frequency of the headphone drivers. It's the frequency where the driver is already most prone to excessive (unwanted) motion and when driven from a source impedance greater than 1/8th the headphone impedance the loss of electrical damping can result in exactly what you describe.

Hey, kinda understand whats being talked about here, and Im sure the answer to my question is here somewhere, but thought i would put it out there anyways.

I have a pair Sennheiser PXC 450s, i play them from my ipod, and laptop computer. I have a feeling the power requirments of the phones is too much for these devices to get the most out of them. The rating is 150/750 for the headphones. Am I correct in my assumption, and would I get more from the headphones is I used a headphone AMp or DACs

@Anon, you bring up a good question and something I should cover better in the article. Most headphones with active noise cancellation, like the PXC 450's, have their own amplifier built in. That's because that's the best way to mix the "anti-noise" signal into the music. So the impedance of active headphones is really the input impedance of their electronic circuit. It's a constant impedance with frequency so the output impedance of the source doesn't matter much.

Put simply: For active headphones you don't have to worry about what's in the article or your source. Adding an amp or headphone DAC won't help any unless the DAC in your laptop or PC has problems.

Thanks NwAvGuy for your comments on the 450's. I understand what you are saying to apply to the 450s when the noise cancelling feature is in effect, however, what happens when it is not engaged. With NC disengaged I clearly hear different levels between sources, such as my PC vs. my iphone. The 150/750 ohm rating, as indicated on Sennheisers website, indicates the 150 ohm level is when NC is not active. A 150 ohm impedance rating would be hard for the iphone to drive right? Would I get more levels if I put an amp infront of the headphones in the "bypass" function of the 450s. Obviously, the NC cct follows the source and any peripheral device you insert, therefor they would have no effect on the output of the phones right?

Thank you for your comments. The pxc 450s operate in a bypass state as "normal" headphones. I believe, by disengaging the noise cancelling cct the phones have an impedance of 150 ohms, and alternately, when the noise canceling feature is activated, the impedance is 750 ohms. I understand, regardless of what you put in front of the noise canceling cct, it will not effect the output of the phones, since the internal cct would override anything previous to it, as you have pointed out.

However, I know from my own use of the phones, there is a distinguishable difference in levels, in both intensity and quality, when used in "bypass" as compared to "normal"(Sennheiser is using the active noise canceling state as normal, and "bypass" as the state when the noise canceling feature is NOT engaged). So, not to be a pest, but if the phones in the "bypass" sate are for all intents and purposes eliminating the noise canceling cct, and if in fact the impedance in the " bypass" state is 150 ohms, would it not be a lot for an iPod to drive? And if so, would a headphone amp help me bring more "life"!to these headphones if being driven by my iPod/iPhone?

Thanks again for the advice.

Oh ya, great stuff here. You actually explain In detail what you mean. It's great. Also, I've read you've angered some people by speaking out against them and their inferior products. I for one love it. The situation around consumers getting misinformed is not getting better, particularly with more and more business being done online. About time someone made them somewhat justified in what they advertise and claim. Keep it up!!!

@Anon re PXC 450. Good point if you use them in bypass mode. 1 volt yields 108 dB with the 450 so, doing the math, they need 1.2 Vrms for the 110 dB peak SPL guideline. That's within the capability of the FiiO E5/E6. It is, however, well beyond what nearly all portable players can manage. So you're correct, to listen "live" levels--especially with highly dynamic music--in bypass mode, you might want an amp for the 450s.

The amp should have a maximum output impedance of 150/8 or 19 ohms and at least 1.3 Vrms of output at 150 ohms. That translates into 3 mW at 600 ohms, 6 mW at 300 ohms, 53 mW at 32 ohms, and 106 mW at 16 ohms. All of this is explained in my More Power? article.

Firstly, I want to apologize! I kinda wrote two comments in a row...didnt even give you a chance to respond. Was not my intention, I just did not realize the first one got sent, becasue, as you point out may happen, I received an error messege. So with that said..

Thanks for your advice. I have read your article on More Power. Again, I have a basic understanding of electronics and electricity, but must admit you lost me in the second half of that article, even after a couple of times through it (at least I tried). Nevertheless, you say I will need 1.2 Vrms to produce the 110 db peak limit, that is essentially considered "loud enough" for most people. You also point out that these limits are within the E5's limits.

So, from this, and the information you have provided in your articles and reponses to me, for me to play my music at the 115 dB level, for my particular heaphones, I will need just under 4 mW for this? With the E5 specs, it would appear with their high and low output power limits of 150 mW @ 16 ohms, and 16 mW @ 300 ohms, the E5 should have more than enough power to drive my headphones (in bypass mode, as we discussed earlier) Any chance this is too much power? Can I damage the headphones if too much power is applied? I understand the amplifier to be a voltage regulater, depending on load, so I suppose the headphones determine the output power of the amplifier (becuase they are the load), and therefor it would not be likly to "overpower" the headphones since they are the determining factor themselves. Am I on the right track at all?

Am I right to assume there should be limited distortion even at these levels, or is that a whole other question for another time?

BTW, I plugged into a AT-PHA10 amplifier, in a big chain store in Tokyo, and, contrary to my previous assumption, the sound was louder even while the noise cancelling feature was engaged. Obviously the other levels, or quality of sound was not as "thick or pure" (as is the case without the amplifier), but nevertheless, the amplifier did impact the NC cct. Any thoughts? I found it interesting. How would it circumnavigate the NC cct.?

Anyways, I rambled long enought....really just trying to maximize my headphone purchase. Bottom line, in your opinion, the E5 would be a good fit for my headphones in order to maximize them?

"Higher impedance headphones also require less current to drive and that typically reduces distortion in multiple ways"Could you explain how though, please? I'm a electrical engineering student but as far as audio electronics go, I've had to dig by myself only, no courses as such. This means that I have a lot of "Okay, but WHY?" questions as I come across info (on the web, at least).So how is distortion related to the current? From what I know, thermal noise increases with resistance so higher headphone impedances should be worse.

@anon, you're on the right track, but perhaps getting bogged down in too many details. You only need to worry about a few numbers/specs. I would wait for my E6 review as it supposedly has some significant improvements over the E5 (and I suspect will be eventually replacing the E5). The maximum voltage of the source partly determines how loud your headphones will get even in active mode. Some sources may not have enough voltage to drive the internal amp to full output. That's likely why they were louder in the store. Yes you can damage headphones with too much power but you don't have to worry about the E5/E6 with your 450s.

@fz, look at most any of my amplifier and headphone DAC reviews you'll find graphs that show THD (distortion) vs Output at different loads. You'll find the same thing in op amp datasheets, etc. In nearly all cases, as the load impedance drops, the distortion rises. For the same output voltage the only thing changing is the amount of current the amplifier has to deliver. More current nearly always means more distortion.

There are multiple reasons for this. First, the output transistors (even in an IC or op amp) become less linear with higher output currents. If you want to geek out, you can look at the transfer curves of transistors and you'll see at higher currents they're less flat (linear). While amps usually have negative feedback, it can only partly correct for the non-linearity.

As an amp puts out more current, there is more audio-related, and sometimes power supply related, ripple on the supply rails. The rails "sag" more with increased output current. This can degrade the performance of the amp--especially a discrete one with relatively poor PSRR.

Nearly all headphones use a 3 wire connection via a 3.5mm or 1/4" phone plug (and some use 3 wire cables). The shared ground degrades crosstalk performance in inverse proportion to the load impedance. Lower impedance loads draw more current creating more drop in the common ground connection which shows up in the opposite channel. Just 50 mOhm of resistance at the headphone jack degrades crosstalk over 30 dB when you drop from 600 ohms to 16 ohm headphones.

Higher current creates greater drops across any series output stage resistors. For resistors insides the feedback loop these higher drops generate greater error signals which indirectly can create other forms of distortion. Amplifiers (even ICs) typically have emitter resistors in the output stage and some designs have series resistors in the outputs for stability or current limiting reasons.

As the load impedance drops many amps will at some point switch from "voltage clipping" to "current clipping". In the first case the amp runs into the power supply rail which limits the peak output. This is usually a fairly clean sort of clipping. But in the second case protection circuits in the amp are triggered to limit the peak current, a DC-DC or regulated power supply may current limit, or some amps simply fall on their face in other ways when asked to deliver current in excess of their design limit. In all of these cases the amp is no longer able to swing close to the normal supply voltage and its output is artificially limited--sometimes in a rather harsh ugly sounding way. Again, if you look at the THD vs Output graphs you'll see most amps I've tested put out a lot less maximum voltage into lower impedance loads at a higher level of distortion.

NwAvGuy, I have a Headstage Arrow 3G paired with the Phonak PFE 112 (balanced armature, 32 ohms). The Arrow has the option to add impedance (0/20/65 ohms if I remember correctly). I heard there are some benefits to adding impedance to the PFEs, but what can you say about this combo?

@Anon, I've never encountered either the Arrow or PFEs myself so I can only speculate. The Arrow is notably lacking for meaningful specs making it even more difficult.

Most B.A. IEMs will have severe frequency response variations with even 20 ohms of output impedance. If someone thinks that makes them sound better, they either have really unusual preferences, the PFE's have some serious flaws, or the PFE's don't behave like most B.A. IEMs (and perhaps are really a dynamic design).

There are measurements done with the Arrow (1G?) by dfkt at ABI. Perhaps he knows more about the design? There was also a manual detailing changes between the 1G-3G but I can't find it right now. I can't find any specs for the Arrow either other than the fact that it uses an Analog AD8397 op amp.

http://rmaa.elektrokrishna.com/index.php?dir=Various/

By the way, here is the claim regarding the added impedance to the PFE's:

"One of my biggest discoveries while reviewing the Phonak, is the impact of adding a 75 ohm impedance adapter to the signal path. The impedance adapter gave the same effect to the Phonak like going from ER4P to ER4S. The guitar crunch comes alive. I like the sound of instruments better, but honestly, some of the warmth of vocal sound has been reduced as well to give a clearer focus to the sound, and this may not be preferable for some genre. Anyway, I recommend this to every Phonak user who has an amp if you want some more focus and sparkle to the sound. The change is subtle, just a nudge to a slightly more analytical direction, but not yet into the realm of extreme focus of the ER4."

@Anon, to each their own. There's no shortage of subjective opinions on headphone audio. Sites like Headfonia are almost entirely based on pure subjectivism. It's no different than a food critic saying he loves a particular restaurant's boiled liver and anchovies. Just because he likes the dish, doesn't mean you will.

Nearly all such comments are only applicable to the one person writing them. They're specific to their music, tastes, preferences, other gear, etc. And a lot of people hear supposedly obvious and large differences that magically disappear if you simply hide the electronics they're listening to. That's how our brains work. See: Subjective vs Objective

Some people like wild EQ settings and artificial DSP processing as well. So it's not surprising some like to mess up the sound of IEMs with a high output impedance. That's all personal preference.

This blog is about trying take a more objective approach that's applicable to a much broader audience. There's lots of objective evidence IEMs deliver much more accurate performance from a near zero ohm impedance source. If someone doesn't like accuracy, that's their individual choice but that doesn't make it the best choice for the next person.

@Anon, There was an interesting discussion on diyAudio about headphones under 16 ohms and there are several. First of all, it makes an even stronger argument for having a "near zero" output impedance as anything much above that is likely to have a significant impact on such headphones. Second, depending on their sensitivity, some headphones require an amp/source with substantial current capability. Finally, some headphone amps could become unstable into such a low impedance load--especially some of the more exotic and/or "high speed" designs.

I plan to test the O2, and upcoming ODA, into 8 ohms and may do a stand alone article on the result. I may combine that testing with comparing real loads to simulated "lab" loads. So look for more here on the topic in the future. I'll probably leave reviews of the headphones themselves to others. But anyone reviewing headphones under 16 ohms needs to be really careful about what sources they use. A source's inability to properly drive such a load may have a substantial impact on the sound quality.

I have a question, if you don't mind. I currently mostly listen to a pair of Koss Pro 4AAs, which are heavy and hot, but to me sound very good, clear from deep bass to harmonics. They are 250 ohms impedance, and get plenty loud enough for me through my sound output on my PC. I also have a pair of Bose headphones and Sennheiser HD238s, which both get louder than I ever listen to, and both sound less clear and more "boomy" to me (although I wouldn't consider either "bad"). Many of the $70 - $300 headphones I have listened to have the same boomy bass and lack of clarity.I am seriously considering a pair of AKG k701s or 702s, which many reviews are good in reference to the image projection of the sound. I believe they are 62 ohms. I would think that at 62 ohms, the amp would put out more power than at 250 ohms. I also wonder if at 62 ohms they would still have good clarity. I have not found a place where I can listen to them. Would the AKGs need to be pushed harder because of things such as magnet placement and strength? Would need a DAC/headphone amp with them, even though they are lower impedance? Should I assume that the perceived clarity and "boominess" are purely subjective; is it just a matter of how I perceive the sound? This would basically make a purchase an expensive flip of a coin.

The Pro 4AA's are ancient but seal really tightly to the head (and least if the ear cushions haven't gone totally flat) which gives them a different sort of bass that many like. More modern headphones that weigh under 18 pounds, and don't feel like a head vice, rest more lightly on the ears. The upside is massively greater comfort but the downside is you lose that tight seal and the eardrum slamming bass that goes with it.

That said, there are still really comfortable headphones that can rattle your eardrums--even better than the Pro 4AAs. One example is the Beyer DT770 Pro 80. I don't think the DT770's are "boomy" but some might but they're like having a subwoofer embedded in your headphones. They have that deep rattle-your-teeth sort of bass when the music calls for it yet they don't add any thickness to male vocals. Bass is very subjective. If you haven't read my DT770 review you might want to as I go into quite a bit of detail about bass, kinds of bass, quality of bass, etc.

The K701/702 (which are no longer made--they've been replaced by the "designer" Q series) are wonderfully comfortable and very well made headphones.They have a very open and spacious sound--although I wouldn't say significantly more so than the Sennheiser HD600/650. The K701 is certainly not boomy. And the bass, what little there is, is nothing like what I remember of listening to Pro 4AAs. The K701 is rather lean in the bass which gives it a kind of "clinical" or "sterile" sound. But from the lower midrange on up they're wonderfully neutral and very detailed. For well recorded music that isn't supposed to have much bass they're hard to beat.

But all that detail also has a downside. The K701 is less than kind to less than perfect recordings--which is to say most music. They shine a very bright spotlight on every flaw, every bit of overprocessing, every trace of clipping in a vocal mic preamp somewhere way up the signal chain, etc. To me that adds up to listener fatigue with anything but true audiophile recordings--of which there are only a hundred or so I can stand to listen to more than a few times. For more on what I think of listener fatigue, see my HD650 review.

As for power it's not that simple. many amps/sources can't put out more power into lower impedance like you would expect them to because they have a high output impedance and/or limited current capability. And the headphone sensitivity (efficiency) plays an even bigger role. The K701/702 (and the new Q701) are fairly demanding of the source. They require quite a bit of current and voltage--something most amps/sources struggle to provide cleanly. They won't play anywhere close to loud enough from most tastes with PC headphone outputs or portable players. Even many portable headphone amps come up short with the 701/702.

For more, see my More Power article for how to figure out what headphones work best with what sources.

After reading several of your articles, it almost sounds as if the Denon AH-D2000s would sound really good to me. That is, of course, if I have a good match to drive them. I have looked at several DAC/headphone amp specs, and have yet to see a good impedance spec. It almost seems like gambling to pick a good match to drive lower-impedance headphones.

I think the D2000 strikes a nice balance and manages to sound "lively" without going too far. My biggest issue is comfort for long term wear but some find them comfortable enough.

It's too bad more companies don't publish the output impedance of their gear. If they don't, and you can't find a reliable review where it was properly measured, it's very much a gamble as you suggest.

With most cans -- except, perhaps ES SM3 IEMs -- I get best subjective performance via the 120-ohm out of my Meier amp. Now, Grado and Beyer suggest an above-zero-Z output jack, IIRC, but even my low-Z Senn IE-8s and AKG-701s sound (SLIGHTLY) better at the 120-ohm O/P. One reasons may be that the FULL range of the volume pot is utilized -- IIRC, vol pots get "more accurate/linear" near their max gain.

I'm guessing you like bloated bass with a frequency response peak around the resonance of the headphone drivers (usually around 100 hz). One man's boom is another man's bass. It's all subjective. Personally, I like accuracy. And that's best achieved with a near-zero ohm output impedance. I also like low distortion and tight bass damping. See:

Hello NwAvGuy,I'm still wondering the differences between impedances. Beyerdynamic DT 880 PRO seems to have excellent frequency response according to headroom, but it doesn't show much difference between the 250 ohm and 600 ohm model.

Is there something happening that can't be seen in the frequency response?

If we have more than enough of power to drive headphones regardless of the impedance, is the higher impedance headphone model always better?

Also, when does the law of diminishing returns kick in? The 250 ohm model would cost me 220€ versus 320€ for the 600 ohm version, which is huge difference and leaves me wondering.

I really respect what you are doing, you will end one insane "hobby" from many people :)

There's no single answer to your question. Ideally if you're undecided between any two headphones, even different versions of the DT880, you need to listen to both and decide for yourself. Even Beyer struggles to answer the very question you asked.

As a guideline, if the same headphone is available in multiple impedance the highest impedance is, at least in theory, capable of the best sound quality assuming you can properly drive it. I explain why in this article. But that doesn't mean it's the best choice for a given person. You might want the flexibility of not always having to use an amplifier or prefer the sound of a lower impedance model. You might also want to save some money as the lower impedance versions tend to be more popular and have a lower street price. The resale value is also likely better on the 250 ohm version if you want something different later.

Unlike amplifiers and DACs, headphones measurements do not accurately predict the sound quality. No headphone is 100% transparent with perfectly flat response, vanishingly low distortion, etc. They all have audible flaws. The measurements can be used as a rough guide--especially if you compare results that were measured in the same way (i.e. Headroom results vs Headroom results). But, because of the pyschoacoustics of the human ear and hearing, the results don't correlate nearly as well as when measuring electronics. A microphone in a plastic head is only a rough approximation.

It's possible, for example, for a headphone that shows slightly less overall bass in a Headroom frequency graph to sound like it has more (or at least better) bass. Comparing high frequency is even less reliable because there's a big resonance problem between about 4 Khz and 12 Khz with most artificial ear/microphone set ups depending on the headphone being tested.

I should probably try to do an article on this topic someday. I'm a big fan of measurements, and they really do predict how electronics will sound, but measurements have some serious limitations when there's a microphone involved--i.e. for measuring headphones and speakers.

Hi NwAvGuy, I was going through your article once again as I keep re-visiting your blog and I read through the Tech Section: Voltage and Current. While I think the analogy is correct, the paragraph makes things sound like Voltage and Current are inversely proportional (Quote: "The two are typically inversely related. High pressure usually means low flow and visa versa. The same is true of voltage and current."). Higher pressure will not mean low flow. Low flow will depend on the hose size or basically the resistance. Given a constant resistance, higher voltage should mean higher current according to Ohms Law.

I could probably put it better, but what I was trying to explain is for a given amount of power low impedance headphones require lower voltage and higher current while high impedance headphones require higher voltage and lower current. So I'm not talking about holding resistance constant as you suggest, but power. In that circumstance voltage and current are inversely proportional. For example 10 mW at 32 ohms requires about 0.5 volts and .018 amps. While 10 mW at 600 ohms requires about 2.5 volts and only 0.004 amps.

Lower impedance means more current is required, and for speakers, that gets expensive because of the relatively high currents involved. For headphones, current is less often a problem than voltage--especially in battery powered gear. Higher impedance often means you need a higher voltage than a lot of gear is capable of.

Thanks for your answers to my questions in your HD650 article. In retrospect it might have been wiser to post the individual questions to their corresponding articles instead of batching them all up in one place - sorry for that.

The hint to look up electrical reactance in Wikipedia was extremely helpful, as it explains a few things and I had never heard of this phenomenon before - I didn't even know such a thing was possible, and I still don't understand "how" it can happen (since this is probably where water pressure analogies reach their limit of usefulness). But at least the "why" (different resistance levels from the magnetic vs. the electric field the coil faces) now makes sense.

I do understand that these are advanced topics that require college-level electrical engineering education to fully grasp - so I'm not really expecting an answer to the how-question above. Back when I was faced with the option of entering the world of EE I was quick to decide on pursuing the path of software "engineering" instead, since it neatly bypasses all these nasty little details. I'm not saying that I regret my choice, but the pros and cons certainly aren't as black and withe any more, ever since discovering your blog. However I do have three more questions with regard to phase shift, that I believe should still be easy to understand for us mere mortals.

1. What is the proper approach to convert the phase shift to a more intuitive metric, like seconds? (phase shift)/360 * 1/frequency?

2. What real world (or audible) implications does the phase shift have? Let's say we have (for simplicity's sake) a recording with a pure 10Hz tone in the first second of it, and a pure 20Hz tone in the second second playing on headphones with 0° phase shift for 10Hz and 90° phase shift for 20Hz. So when the tone (=frequency the coil is swinging at => the signal's ac-voltage?) changes right after the first second, the coil starts swinging at the new tone's frequency (ac-voltage), but still continues to receive the previous tone's current-level for the next 12.5ms (according to the calculation above: 90/360 * 1/20 ) which would result in the wrong amount of driver excursion for this time?

3. I read above that this phase shift indicates that a headphone is harder to drive, but how so? A regular amp (that has no logic of its own and no knowledge of the headphone's phase shift in advance – like the O2 or E5) can't really pre-empt this phenomenon, so how can it correct for it, in order to drive the headphones properly? Or is this something that the negative feedback takes care of? Shouldn't this really be more of an indicator of poor engineering for the headphone, instead of a metric for how "hard" it is to drive?

1 When the time shift is less than one cycle at the measurement frequency it's measured in degrees. When it's greater than one cycle you have to use time (sometimes called "group delay").

2 I don't understand what you're asking. But I think the root of the problem is you're still not interpreting phase correctly. See the next answer.

3 The phase shift you see in the graphs above does NOT mean the driver is moving out of phase with the input signal. All it means is the current flow through the driver is out phase with the voltage presented to the driver. The more out of phase the voltage and current are, the more reactive the load is. For reasons that are far to complex to go into here, more reactive loads are more challenging for amplifiers to deal with.

You might try looking up Power Factor (PF). It's essentially the same thing. A 100 watt motor is a more difficult load on the power grid than a 100 watt incandescent light bulb because the motor is more reactive and has greater phase shift between the voltage and current.

Hey NWAVGuy I had a quick question about ohm impedance because I got in a bit of a dust up with some Nuforce gear loyalist at Headfi and wanted to get your take on it.

Some guys are asserting that output impedance doesn't matter with planar magnetic headphone and were using old datasheets from Yamaha orthos to prove it.... my thing was that that those Yamaha orthos were built well ahead of the iPod and were 150 ohm headphones.... The other guy arguing that it was okay was using a an old integrated receiver which has a significantly higher voltage than your average small scale semi portable headphone amp... so I didn't see where either of those arguments were relevant to whether a 10ohm output impedance on a NUForce HDP amp will cause problems with a 40 ohm Hifiman headphone.

My thinking is that Hifiman and Audeze essentially have a sort of passive mechanical damping on their drivers (the magnet arrays on either side) which is why they are so current hungry even as low impedance cans... because the magnets are constantly exerting pressure on the diaphragms.... so they require much more current to sufficiently vibrate the diaphragm and get music.

If you have a headphone with mechanical damping you may not see sever bass damping. But the problem that I see, is that it needs a lot of current and its being supplied current by a relatively low voltage high output impedance amp. Couldn't you end up with insufficient current supply? And beyond that isn't there more risk to that than simply not getting loud enough?

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